This study describes the structural evolution of the Klondike Schist and the structural setting of mineralized mesothermal veins from which over 500 tonnes of placer gold has been derived. The Klondike Schist was emplaced as a series of thrust slices on top of a structural stack that includes at least three additional thrust slices. A distinctive set of mesoscopic structures, particularly a set of recumbent folds, formed during thrust emplacement. These folds have a general southeast trend, and deform thrust-emplaced ultramafic rocks. Underlying thrust panels contain folds that resemble these thrust-related structures, but have a consistent northeast trend. The Klondike Schist may have been rotated about a vertical axis during the latter stages of thrusting along phacoidal cleavage zones. Extensional sites in post-thrust kink folds and faults host mesothermal gold veins. Hence, gold mineralization postdated thrust stacking. Normal faults offset mesothermal veins, and host late-stage hydrothermal alteration zones.

Description and analysis of the geology of quartz veins containing gold ores in the Yukon Territory.

Four main styles of quartz veining are recognized in the Klondike District. These include foliaform and discordant mesothermal quartz veins in schistose metamorphic rocks, quartz-carbonate veins in altered ultramafic rocks and greenstones, epithermal chalcedony veins cutting Eocene igneous and sedimentary rocks, and low-temperature epithermal veins associated with intensely altered Plio-Pleistocene White Channel Gravel deposits. Foliaform mesothermal quartz veins are invariably barren; however discordant mesothermal veins locally contain visible gold. Both styles of epithermal veins in the Klondike contain at least geochemically anomalous levels of gold. The complex history of hydrothermal activity in the Klondike has led to considerable confusion about the nature of veining in this area. The results of this study provide a preliminary framework within which to evaluate the various prevailing theories regarding the major sources of gold in the Klondike placer deposits.

A detailed evaluation of structure and alteration related to gold- and silver-rich, base metal-bearing veins was completed at the Skukum property as part of the 2002 mineral exploration program. The structural setting is an east-trending sinistral strike-slip system bounded by the Berney Creek and Goddell faults to the south and north, respectively. The deposit comprises northeast-trending quartz-sulphide mineral shear veins that formed during syn-tectonic intrusion of rhyolite and andesite dykes related to the Eocene Mount Skukum caldera complex. A genetic relationship between mineralization and certain rhyolite dykes is indicated by patterns of alteration and mineralization. Dilational, northeast-trending structures interconnect and splay off the controlling faults, and host extensional quartz-sulphide mineral veins. At Skukum Creek the main gold-silver-bearing minerals are electrum and freibergite, which precipitated with late galena-stibnite mineralization, whereas refractory gold in arsenopyrite is the main style at Goddell. A geological model is proposed that facilitates identification of prospective structures within the property.

Gold mineralization at the Mt. Skukum deposit occurs in nearly vertical quartz-carbonate veins which crosscut flat-lying andesites with a NNE trend. The mineralized veins represent the second stage of a two stage hydrothermal system, the first of which resulted in emplacement of thin chalcedonic veinlets. These two stages of veins are probably indicative of an evolving hydrothermal fluid rather than being representative of two separate events. Vein emplacement is one of the latest of a series of events which began with volcanism, producing felsic and andesitic volcanic rocks which overlie basement in this area. Subsequent periods of tectonism produced large faults along which rhyolitic dykes were emplaced. Continued tectonism resulted in reactivation of old faults along which andesitic and dacitic dykes were injected, crosscutting rhyolite dykes in many cases. As volcanic activity waned, the faults remained active, leaving zones of high permeability which acted as conduits for the still active hydrothermal circulation. Veins appear to have been emplaced at low temperature in a circulating hydrothermal system driven by a heat source at depth associated with dykes present in the area. Circulating hydrothermal fluids may have leached gold from the surrounding andesitic volcanics during propylitization. Permeability may have been controlled by faulting, brecciated flow tops and bottoms, and lapilli tuff horizons. Gold was precipitated in highly permeable conduits, such as the Main Fault Zone and breccia bodies.

Structure imparts a significant control on the distribution of Carlin-type gold mineralization recently discovered in the Nadaleen trend, Yukon. An improved understanding of the structural framework for gold mineralization is essential for continued exploration success and interpreting ore fluid controls. Structural observations from the Osiris cluster of the Nadaleen trend indicate that NW-verging F1 folds were refolded in response to later SSW-NNE directed contraction. F2 folds have a subvertical ESE-striking axial plane with subvertically plunging axes on steep F1 limbs and subhorizontal fold axes in shallow F1 limbs. F2 folds have a pervasive axial planar cleavage that is recognized regionally. The steeply dipping Osiris and Nadaleen faults appear to cut all folds. Mineralization is spatially associated with later NW-striking faults in the Conrad zone. Much of the folding within the mineralized Conrad limestone is synsedimentary and its geometry reflects its emplacement as an olistostrome.

Compositional studies have been undertaken on gold particles recovered from hypogene ore, eluvial material and placer samples in and around the Klaza property. These data have been correlated with previous descriptions of in situ mineralization to elucidate placer-lode relationships and systematic change in gold compositions between porphyry and epithermal environments. Gold alloy from the porphyry environment is Ag-poor with respect to Au formed in later stage veins. Silver, and to a lesser extent Cu, have been the main discriminants for inferring the source of Au within the placers, and in general, vein mineralization is a more important source-type than porphyry mineralization. The signature of Pb-Bi-Te previously identified in the inclusion suites of Au grains from Nucleus/Revenue, Casino and Sonora Gulch has also been identified at Klaza, demonstrating that generic compositional signatures can underpin a robust exploration methodology. The relative sizes of porphyry and epithermal footprints of detrital Au together with their respective compositions are important considerations when targeting Cu-Au systems.

Gold occurrences in the upper Hyland River valley form a 50-km-long belt that is considered to be the easternmost portion of the Tombstone Gold Belt (TGB). Mineralization is thought to have a genetic association with nearby Cretaceous plutons, which were important in the formation of most mineralization in the TGB. However, an evaluation of the Hyland River occurrences indicates that evidence supporting an intrusion-related gold model is mostly lacking. Plutons and dykes do not occur in the vicinity of the gold occurrences; there are no obvious zones of hornfels; contact metamorphic minerals and skarns are mostly absent; there is no known mineral or metal zonation typical of intrusion-related systems; and aeromagnetic lows result from massive, variably altered quartz grit and conglomerate and not from unroofed `low-mag' intrusions. Mineralization consists of four types: 1) disseminated pyrite and arsenopyrite in altered grit; 2) quartz-arsenopyrite veins; 3) quartz-pyrite-galena veins; and, 4) massive arsenopyrite veins. Auriferous quartz veins have characteristics similar to orogenic gold veins, and thus potentially relate to regional metamorphism and large structural features.

La Teko Resources Ltd. acquired the Scheelite Dome gold property from Kennecott Canada Exploration Inc. in 1998. Kennecott had explored the central Yukon property since 1994. The strongly deformed Yusezyu Formation of the Upper Proterozoic-Lower Cambrian Hyland Group underlies the property and is intruded by unfoliated mid-Cretaceous granitic stocks, dykes and sills of the Tombstone Plutonic Suite (TPS). Vein-type (both metasediment- and granite-hosted), skarn and replacement mineralization on the property is associated with the TPS intrusives. Mapping, trenching and drilling by Kennecott identified numerous structurally controlled metasediment-hosted zones of mineralization within an east-west 3.5 km by 1.4 km > 40 ppb, gold soil anomaly. In 1997, a 13 hole 1052 m reverse circulation drill program tested the gold soil anomaly with the two best holes returning weighted averages of 0.48 g/t gold over 29 m (RC97-4) and 0.41 g/t gold over 61 m (RC97-11). In 1998, a seven hole 1268 m diamond drill program by La Teko tested targets defined using a combination of soil and rock gold anomalies, geological structures and chargeability and resistivity anomalies. Results included intersections of 1.04 g/t gold over 14.9 m, 1.07 g/t gold over 12.1 m, and 3.67 g/t gold over 7.7 m.

We have determined the major and trace element composition of approximately 2700 gold particles from 21 gold-bearing mesothermal quartz veins and 35 placer gold samples from the Klondike District in western Yukon Territory. Measured Au, Ag, Cu, and Hg contents were used to define a characteristic geochemical signature for each of the vein samples. These signatures were then compared with the various compositional populations that we have distinguished within each of the placer samples. Preliminary conclusions derived from the study include: 1) placer gold in both recent stream deposits and in the Plio-Pleistocene White Channel Gravels is detrital in origin; 2) the placer gold is mainly, if not entirely, derived from mesothermal quartz veins; 3) all lode sources for the placer gold have not yet been located; and 4) composition data can be used to link placer gold to its specific lode source.